Various techniques for stabilizing the frequency of lasers exist in prior art. For example, in U.S. Pat. No. 3,742,382 (Smith), an apparatus for stabilizing a laser to a gas absorption line is disclosed. The apparatus includes a gaseous frequency standard cell through which a first component of the output beam is passed in a first direction and then detected. A second component of the beam is also passed through the cell in a second direction slightly less than 180.degree. from the first direction. The detected signal as a function of frequency has a narrow peak centered on the absorption line of the gas. The width of this peak is the homogeneous line width of the absorbing gas, which is typically much narrower than the Doppler-broadened absorption line. The detected signal is then applied through a feedback loop to control the tuning of the laser resonator.
In "Optical Heterodyne Saturation Spectroscopy", J.L. Hall, L. Hollberg, T. Baer, and H.G. Robinson, Applied Physics Letters 39(9), page 680, November 1981, an optical heterodyne saturation spectrometer is disclosed. The spectrometer includes a single frequency laser output which is divided into a probe beam which is phase modulated and a saturating beam which is frequency offset and chopped by an acousto-optic modulator. The two beams are passed in opposite directions through an iodine cell whose pressure is controlled. The signal-bearing probe beam is detected by a fast photodiode.
In "High Sensitivity Non-linear Spectroscopy Using A Frequency-Offset Pump", J. J. Snyder, R. K. Raj, D. Bloch, and M. Ducloy, Optics Letters, Vol. 5, No. 4, Page 163, April 1980, a simple method for eliminating coherent background noise and fluctuations from saturation-spectroscopy signals is disclosed. According to this technique, the output beam of a laser is split by a polarizing beam splitter into two beams. The pump beam is frequency shifted and chopped by an acousto-optic modulator and passed through an iodine cell. The probe beam is then passed through the iodine cell in the opposite direction and detected by a photodiode. The photodiode output at the acousto-optic chopping frequency is amplified and demodulated by a lock-in amplifier.
In "Modulation Transfer Processes in Optical Heterodyne Saturation Spectroscopy", J. H. Shirley, Optics Letters, Vol. 7, Page 537, November 1982, the transfer of modulation from a phase modulated laser beam to an unmodulated, oppositely running beam in a non-linear resonant gaseous medium is discussed. Heterodyne detection of the transferred modulation reveals multiplex patterns that are distinct for the two mechanisms which account for the transfer, namely, modulated hole burning and reflection from an induced population grating.